HYDROGEOCHEMICAL AND ENVIRONMENTAL ISOTOPE CHARACTERIZATION OF THE CO2 SPRINGS ALONG THE BONGWANA
FAULT, ITS IMPACT ON FRESH WATER RESOURCES AND IMPLICATIONS FOR CARBON CAPTURE AND STORAGE (CCS)
IN SOUTH AFRICA
Mzikayise Nkwane
Submitted in fulfilment of the requirements for the degree of Master of Science in Hydrogeology
Discipline of Geological sciences
School of Agricultural, Earth and Environmental Science College of Agriculture, Engineering and Science
University of KwaZulu-Natal Durban
South Africa
March 2018
As the Candidate’s Supervisor, I have approved this dissertation for submission.
Signed: --- Name: ---Date: ---
PREFACE
The research reported in this dissertation was completed by the candidate while based in the Discipline of Geological Sciences, School of Agricultural, Earth and Environmental Sciences within the College of Agriculture, Engineering and Science, University of KwaZulu-Natal, South Africa. The research was financially supported by the South Africa Centre for Carbon Capture and Storage (SACCCS).
The contents of this work have not been submitted in any form to another university and, except where the work of others is acknowledged in the text, the results reported are due to investigations by the candidate.
Declaration – Publications
Details of contributions to publications that form part and/or include research presented in this dissertation are:
Nkwane, M. and Demlie, M. (2017). Characterization of the CO2 springs associated with the Bongwana Fault: its impact on surface water and groundwater quality and implications on carbon capture and storage (CCS) in South Africa. 15th Biennial conference of the Groundwater Division (GWD2017: Change- Challenge-opportunity). 16-18 October 2017, Stellenbosch, South Africa.
Nkwane, M. (2017). Analysis and modelling of the impacts of CO2 springs associated with Bongwana fault on fresh groundwater and surface water quality and implications on carbon capture and storage (ccs) in South Africa. 5th Biennial Carbon Capture and Storage Conference 2017. 18-19 October 2017 Coastlands Hotel & Convention Centre Umhlanga, KZN, South Africa.
DECLARATION: PLAGIARISM
I, Mzikayise Nkwane, declare that:
I. The research reported in this dissertation, except where otherwise indicated or acknowledged, is my original work;
II. This dissertation has not been submitted in full or in part for any degree or examination to any other university;
III. This dissertation does not contain other persons’ data, pictures, graphs or other information, unless specifically acknowledged as being sourced from other persons;
IV. This dissertation does not contain other persons’ writing, unless specifically acknowledged as being sourced from other researchers. Where other written sources have been quoted, then:
a. Their words have been re-written but the general information attributed to them has been referenced;
b. Where their exact words have been used, their writing has been placed inside quotation marks, and referenced;
V. Where I have used material for which publications followed, I have indicated in detail my role in the work;
VI. This dissertation is primarily a collection of material, prepared by myself, published as journal articles or presented as a poster and oral presentations at conferences. In some cases, additional material has been included;
VII. This dissertation does not contain text, graphics or tables copied and pasted from the Internet, unless specifically acknowledged, and the source being detailed in the dissertation and in the References sections.
__ ____________________
Signed: Mzikayise Nkwane Date: 05 March 2018
ABSTRACT
Natural CO2-rich springs at the Bongwana area in Eastern South Africa emanate from three sites along an 80 km long North-South trending Bongwana Fault. The geological unit that outcrops along the extent of the Fault are the Dwyka Group rocks that are made up of mainly tillites and subordinate sandstones, shales and conglomerates. The objectives of this M.Sc. study is to characterize these CO2-rich springs and assess their impacts on shallow groundwater and surface water chemistry and consequently to understand the implication of a failed CCS facility. Groundwater and surface samples were collected both at CO2 emission and CO2-free springs, boreholes and streams around the length of the studied fault zone for the analyses of major ions, trace elements and environmental isotopes. Additionally, specific electrical conductivity (EC), total dissolved solids (TDS), pH, temperature, dissolved oxygen (DO), redox potential (Eh), total alkalinity, CO32- and HCO3- concentrations were determined onsite.
The results indicate that all the travertine cone springs located near Umtamvuna River are characterized by Na-Ca-Mg-HCO3 water types, while boreholes from shallow groundwater and river samples show Ca-Na-Mg-HCO3 types. Stable isotope (δ18O and δ2H) composition of the travertine cone springs shows a major negative shift from the meteoric water lines with δ18O and δ2H values ranging from -7.78 to -6.52 ‰ and -21.5 to -17.9 ‰, respectively. While, the stable isotopic composition of shallow groundwater reflects local and modern meteoric recharge. These observations indicate that the reservoir and source of recharge for the deep circulating groundwater are different from the shallow groundwater. Based on onsite hydrogeological, hydrogeochemical, and environmental isotope observations, a hydrogeological conceptual model is proposed, which states that the groundwater recharge for deep circulating groundwater is to the west of the Bongwana fault, located at a higher altitude. From these altitudes, groundwater percolates through deep fractures and faults to greater depths. As groundwater percolates through the rock, it interacts with minerals and the initial recharge chemistry and isotopic composition is altered along the groundwater flow paths. At depth, groundwater dissolves carbonate rocks and as a result CO2 is generated.
The dissolution of CO2 in groundwater further drives the leaching of the formation minerals along the flow path. Near the surface, CO2 exsolves and travertine mainly composed of calcite, amorphous silica and iron hydroxides is formed. Geochemical inverse modelling and bivariate correlation among groundwater hydrochemical parameters for travertine springs indicate that the major geochemical processes that are responsible for the observed chemical composition are the dissolution of calcite, dolomite, Pyrite, Goethite, K-feldspars, fluorite, albite and sylvite and the precipitation of calcite, amorphous silica, iron hydroxide, iron carbonates, kaolinite and CO2 gas.
The carbonate minerals are attributed to the dissolution of carbonate rocks at depth.
Feldspars are common from the Dwyka Group Diamictites, whereas the plagioclase feldspar (albite) is probably originating from the recharge area outside of the Dwyka group or leached from the granitic and metamorphic fragments contained within the Dwyka tillites. These inverse modelling results are supported by the saturation indices (SI) for calcite and dolomite in these springs which range from 0.74 to 0.82 and from 0.24 to 1.35, respectively indicating oversaturation with respect to these minerals and subsequent precipitation out of the aqueous solution. The precipitation of calcite, amorphous silica and iron carbonates were confirmed by the XRF, XRD and thin section results of the travertine rock samples. Acidic pH conditions (5.5), elevated TDS (5937 ppm), EC (3271 mS/m) and high trace metals concentration were detected in all CO2 emission sites compared to CO2 free streams, springs and boreholes. These results clearly show the impacts of CO2 on groundwater and surface water quality within the vicinity of emission points. Therefore, it appears that natural CO2 emission along the Bongwana fault have impacted the ambient groundwater and surface water quality at the emission sites rendering it unfit for human consumption due to elevated concentration of dissolved constituents above safe drinking standards. The implication of this to CCS in South Africa is the fact that any unintended CO2 leakage into fresh groundwater and surface water resources from a failed subsurface storage facility may impact freshwater resources. Thus, strict scientific site selection protocols and properly designed monitoring systems are required to minimise the risk.
Key Words/Phrases: Bongwana Fault, CO2-rich springs, Carbon capture and storage, Environmental isotopes, Hydrochemistry, Inverse modelling, South Africa.
ACKNOWLEDGMENTS
The author would like to acknowledge the South African Centre for Carbon Capture and Storage (SACCCS) for financing the research and Dr Demlie Molla, my research supervisor, for his contribution in giving guidance, review and comments during planning of the research, data collection during field work and reviewing the research dissertation.
GCS (water and Environmental Consultants) (Pty) Ltd-Durban office is acknowledged for allowing the author to use their facilities during desktop study and the compilation of the research dissertation and also for the time they allowed the author to take off from work and focus on this research. Many thanks to Hendrik Botha, Callie Pickering and Pieter Labuschagne for their comments and suggestions which greatly improved the quality of the dissertation in terms of scientific knowledge and interpretation of data.
CONTENTS PAGES
1. CHAPTER ONE: INTRODUCTION ... 1
1.1BACKGROUND TO THE RESEARCH ... 1
1.3HYPOTHESIS ... 2
1.4RESEARCH QUESTIONS ... 3
1.5RESEARCH AIM AND OBJECTIVES ... 3
2. CHAPTER 2: DESCRIPTION OF THE STUDY AREA ... 6
2.1LOCATION OF THE STUDY AREA ... 6
2.2CLIMATE AND DRAINAGE ... 8
2.3GEOLOGICAL SETTING ... 10
2.4HYDROGEOLOGICAL CONDITIONS ... 13
2.4.1 Aquifer types ... 13
2.4.3 General groundwater quality ... 14
3. CHAPTER 3: LITERATURE REVIEW ... 15
3.1CARBON DIOXIDE EMISSIONS AND CLIMATE CHANGE ... 15
3.1.1SOURCE OF CO2 ... 16
Anthropogenic sources ... 16
Natural sources ... 16
3.1.2 Migration of CO2 in the earth’s crust ... 18
3.1.3 Impacts of dissolved CO2 on water chemistry ... 19
3.1.4 Technique to curb CO2 release: Carbon Capture and Storage (CCS) ... 21
4) Transportation ... 23
5) Storage ... 24
3.2PREVIOUS STUDIES ON THE BONGWANA FAULT ... 25
4. CHAPTER 4: RESEARCH METHODOLOGY ... 26
4.1DATA COLLECTION ... 26
4.1.1 Desktop study ... 26
4.1.2 Literature review ... 27
4.1.3 Data consolidation ... 27
4.2FIELD ASSESSMENT ... 27
4.2.1 Preliminary field assessment ... 27
4.2.2 Hydro-census and sample collection ... 28
4.2.3 Field measurements ... 28
4.4DATA ANALYSIS, INTERPRETATION AND CONCEPTUALIZATION ... 32
5. CHAPTER 5: RESULTS ... 34
5.1FIELD MEASUREMENTS ... 34
5.2WATER QUALITY AND HYDROCHEMISTRY ... 38
5.3 Linkages among geochemical parameters of groundwater ... 41
5.4 Saturation indexes of groundwater and surface water with respect to selected minerals ... 45
5.5GEOCHEMICAL COMPOSITION OF TRAVERTINE ROCK SAMPLES ... 49
5.6HYDROGEOCHEMICAL INVERSE MODELING ... 53
5.7HYDROGEOCHEMICAL FORWARD MODELING... 58
5.8ENVIRONMENTAL ISOTOPE SIGNATURES ... 61
6. CHAPTER 6: DISCUSSION ... 64
6.1CO2 IMPACTS ON GROUNDWATER AND SURFACE WATER... 64
6.2EVOLUTION OF GROUNDWATER HYDROCHEMISTRY ... 64
6.3ORIGIN OF CO2 ... 65
6.4ORIGIN AND AGE OF GROUNDWATER. ... 66
6.5IMPLICATION FOR CCS IN SOUTH AFRICA AND THE NEED FOR ROBUST MONITORING... 67
6.7LESSONS LEARNT FROM THE BONGWANA STUDY ... 68
6.8SHALLOW GROUNDWATER MONITORING SYSTEM ... 68
6.9HYDROGEOLOGICAL CONCEPTUALIZATION OF THE OCCURRENCE AND CIRCULATION OF SHALLOW GROUNDWATER AND DEEP CO2-RICH GROUNDWATER ALONG THE BONGWANA FAULT ... 69
7. CHAPTER 7: CONCLUSIONS AND RECOMMENDATIONS ... 71
7.1CONCLUSIONS ... 71
7.2RECOMMENDATIONS ... 73
8. CHAPTER 8: REFERENCES ... 74
LIST OF FIGURES Figure 2-1: Locality map of the study area ... 7
Figure 3-1: Schematic representation of the processes involved in CCS. ... 22
Figure 4-1: A flow chart showing the processes followed during the course of this research ... 33
Figure 5-1: Graph of the electrical conductivity versus the pH for CO2 emission sites ... 35
Figure 5-2: Graph of the electrical conductivity versus the pH for sites with no CO2 emissions. ... 35
Figure 5-3: Piper plot of the major cations and anions composition for groundwater and surface water samples. The red colour shows CO2-rich sites and yellow colour shows CO2-poor sites. ... 40
Figure 5-4: Correlation between HCO3-and Ca2+ in groundwater. ... 42
Figure 5-5: Correlation between HCO3 and Mg2+ in groundwater. ... 43
Figure 5-6: Correlation between HCO3-and Na+ in groundwater. ... 43
Figure 5-7: Correlation between SO4-and Ca2+ in groundwater. ... 44
Figure 5-8: Correlation between Cl- and Na+ in groundwater. ... 44
Figure 5-9: Correlation between SO4-and Na+ in groundwater. ... 45
Figure 5-10: Plot of saturation indexes with respect to some minerals in water. A: plot of SI calcite vs TDS. B: Plot of SI dolomite vs TDS. C: Plot of SI Aragonite vs TDS. D: plot of SI Gypsum vs TDS. E: plot of SI anhydrite vs TDS. 48 Figure 5-11: Thin section photos taken under polarised microscope. ... 50
Figure 5-12: Ternary diagram of the travertine sample oxide composition (diagram drawn after Torres et al., (2000)). ... 51
Figure 5-13: X-ray diffractogram of the travertine rock sample ML3. ... 52
Figure 5-14: X-ray diffractogram of the travertine rock sample ML4. ... 52
Figure 5-15: Graphical representation of the inverse model for travertine cone spring, sample number BGN-12. ... 55
Figure 5-16: Graphical representation of the inverse model for travertine cone spring, sample number BGN-15. ... 56
Figure 5-17: Graphical representation of the inverse model for travertine cone spring, sample number BGN-16. ... 57
Figure 5-18: Forward model graph. The lines show the steps of the forward model and the dots at the end of the lines show the average concentration measured from the travertine springs. ... 59
Figure 5-19: Relationship between oxygen and hydrogen isotopes in shallow groundwater, surface water and travertine cone springs in the study area. Local Meteoric Water Line (LMWL) is shown for comparison. ... 63
Figure 6-1: Conceptual diagram indicating groundwater recharge and inferred flow direction. ... 70
LIST OF TABLES Table 5-1: Insitu water quality parameters ... 36
Table 5-2: Major cations and anions ... 39
Table 5-3: Hydrochemical facies (water types) for water samples analysed in the study area. ... 40
Table 5-4: Saturation indexes ... 47
Table 5-5: XRF analysis results of the travertine rock samples ML3 and ML4. ... 51
Table 8-1: GRIP boreholes (Hydrochemistry and water levels) ... 82
LIST OF APPENDICES
APPENDIX A: BOREHOLE INFORMATION FOR THE GRIP DATA BASE OF THE STUDY AREA (DW&S, 2016) ... 82 APPENDIX B: TRACE ELEMENT COMPOSITION FOR GROUNDWATER AND SURFACE WATER SITES ... 83 APPENDIX C: TRACE ELEMENT COMPOSITION OF THE TRAVERTINE CONE ... 85
LIST OF ACRONYMS
CCS : Carbon Capture and Storage DO : Dissolved Oxygen
EC : Electrical Conductivity
GRIP : Groundwater Resource Information Projects IAP : Ionic Activity Product
KZN : Kwazulu Natal
MAP : Mean Annual Precipitation MBGL : Meters below Ground Level Mg/l : Milligrams per litre
ml : Millilitre
mS/m : Millisiemens per Meter N/A : Not Applicable
ORP : Oxidation Reduction Potential PCSP : Pilot Carbon dioxide Storage Project Ppm : Parts Per Million
SACCCS : South African Centre for Carbon Capture and Storage SANS : South African National Standards
SI : Saturation Index TALK : Total Alkalinity
TDS : Total Dissolved Solids TMS : Table Mountain Sandstone TU : Tritium Units
WRC : Water Research Commission
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1. CHAPTER ONE: INTRODUCTION
1. 1 Background to the research
Various comprehensive studies on global climate change have concluded that recent increases in the average global temperature are most likely the effect of the increase in concentration of carbon dioxide (CO2) and other greenhouse gases in the atmosphere.
The increase in CO2 concentrations in the atmosphere, as a result of anthropogenic activities, has been tagged as a major cause of global climate change. This is because CO2 is a primary greenhouse gas and the increase in its concentration in the atmosphere has significant impacts on climate change.
Carbon capture and storage in deep geologic formations, which aims at capturing and injecting CO2 into deep subsurface rock formations for long-term storage, is being explored worldwide as an option to reduce the impact of CO2 emissions on global climate change and human health (Dafflon, et al., 2012; Benson and Cook, 2005; IPCC, 2007).
In South Africa, the Carbon Capture and Storage (CCS) activities are undertaken by the South African Centre for Carbon Capture and Storage (SACCCS). SACCCS is undertaking pilot studies to investigate the feasibility of capturing and injecting CO2 in deep saline aquifers in South Africa. Pilot studies are being undertaken in the Zululand basin in KwaZulu-Natal and the Algoa Basin in the Eastern Cape.
The major concern with carbon dioxide sequestration in deep geologic formations is the unintended leakage of CO2 from the storage reservoir into the fresh groundwater aquifers and the resultant associated impacts on groundwater quality (Yang, 2014).
Research shows that the dissolution of CO2 in groundwaterlowers pH and may further mobilise naturally occurring trace metals and ions that are commonly adsorbed on to or contained in sediments (Daflon, 2012; Kharaka, 2009; Smyth, 2008; Benson, 2006;
CO2GeoNet, 2011).
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Since CO2 sequestration is a new concept in South Africa, groundwater and surface water monitoring along the Bongwana fault was considered as an excellent analogue to study the impacts of a failed CO2 storage site, as natural CO2 emits at various points along the fault. The two leakage scenarios from a failed CCS facility under consideration are the abrupt leakage through injection well failure or leakage from an abandoned well, and gradual leakage through undetected faults, fractures or wells.
Therefore, this research seeks to characterise groundwater that is associated with the CO2 springs in terms of its hydrochemical and isotope composition and demonstrate, with field and laboratory evidence, the risks associated with dissolution of carbon dioxide in fresh water resources. The research will form basis for the selection of the most likely geochemical changes that can be monitored around the CO2 underground storage facility that can indicate the intrusion of CO2 in groundwater and surface water resources.
1.2 Problem statement
The South African Centre for Carbon Capture and Storage (SACCCS) is investigating the feasibility of undertaking a pilot CO2 capture and storage study in deep geologic formations in South Africa. The challenge facing the pilot project is the lack of monitoring data related to impacts of CO2 on fresh water resources if an unforeseen release of CO2 from an underground storage facility occurs.
The area along the Bongwana fault was suggested by SACCCS as a relevant analogue site to undertake groundwater monitoring and to understand the impacts of CO2 release on freshwater resources. However, there is insufficient information and data showing the interaction of CO2 emanating along the Bongwana Fault with fresh water resources.
1.3 Hypothesis
The CO2 springs associated with the Bongwana fault provides an excellent analogue to study the impacts of CO2 leakage from a failed underground storage facility into fresh groundwater and surface water resources under the two leakage scenarios, namely;
sudden leakage through injection well failure or leakage up an abandoned well, and gradual leakage, through undetected faults, fractures or wells.
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1.4 Research questions
In order to prove the above mentioned hypothesis, the following research questions were generated:
What is the ambient groundwater and surface water quality in the study area without the impacts of CO2?
What are the most likely geochemical processes that contribute to the evolution of groundwater and surface water chemistry?
Can the introduction of CO2 in groundwater and surface water in the study area affects the ambient water chemistry? And how?
What are the geological, hydrogeological and hydrochemical characteristics within the study area?
Are there any other sources of contaminants within the study area that could potentially result in the same impacts as CO2?
1.5 Research aim and objectives
The aim of this research is to assess the impacts the CO2 springs have on fresh groundwater and surface water quality in the Bongwana area and to supply a scientific understanding of the risks to fresh water resources associated with the pilot CCS in South Africa as well as to recommend mitigation measures to minimize the risk.
The main objectives of this M.Sc. Research are:
To characterise the CO2 springs associated with the Bongwana fault.
To assess their impacts on fresh groundwater and surface water quality by assessing the hydro-geochemical changes in groundwater as a result of the introduction of CO2 in groundwater and surface water.
To understand the possible source of the chemical parameters in groundwater and recharge for groundwater for the Bongwana area.
Comment on the feasibility, potential risks of the CCS activities in the Zululand Basin of South Africa.
Supply recommendations on the groundwater and surface water monitoring plans for the pilot CCS site in South Africa.
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Comment on the chemical determinants that can be used as indicators of CO2 leakage from the storage site into the fresh water aquifer.
1.6
Dissertation structureChapter one: This chapter provides an overview of the research project including statement of the problem and research aims and objectives.
Chapter two: This chapter presents the geographic location of the study area and discusses the climatic, geological and hydrogeological conditions of the study area. The information used in this chapter was gathered through the assessment of the previous geological and hydrogeological reports conducted within the study area as well as geological and hydrogeological maps published for the area.
Chapter three: this literature review chapter discusses the sources of carbon dioxide emissions and its impacts as a greenhouse gas and further discusses the processes that are involved in the capturing, transporting and sequestration of carbon dioxide as an effort to curb its emissions. Furthermore, the chapter discusses the migration of CO2 gas within the earth’s crust and how it affects groundwater and surface water quality. Most of the information used in this chapter is based on papers and reports from previous studies conducted around the world where natural CO2 sites have been used to understand CO2 migration within the earth’s crust and the impacts on groundwater and surface water associated with the dissolution of CO2 in water.
Chapter four: this chapter describes the methodology used to in undertaking the research project. It describes a sequence of tasks completed in order to make sure the research was a success. These tasks include the gathering of information on a desktop level, field assessment, laboratory analysis as well as data analysis and interpretation. All the information gathered during each task is used to inform the study.
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Chapter five: This chapter presents all the results obtained during the field assessment and laboratory analysis. The information is displayed in the form of tables, graphs and diagrams. The field assessment data include the in-situ water quality parameters, groundwater level measurements in boreholes, field titration, among others. The laboratory data includes hydrochemistry, environmental isotopes, petrography, and geochemical data on travertine composition.
Chapter six: discusses the findings of the study and presents the interpretation based on the results obtained. The information discussed in this chapter is based on the analysis of information collected during the desktop survey, field assessment and laboratory analysis. Based on the overall findings of the study, a conceptual model is proposed that illustrates the origin, occurrence and circulation of the CO2 rich spring along the Bongwana Fault.
Chapter seven concludes with the main findings of the research, the outcome of which presents technical recommendations for groundwater monitoring requirements related to CCS in deep underground storage facilities. Finally, all references used in the preparation of the dissertation are listed under list of references.
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2. CHAPTER 2: DESCRIPTION OF THE STUDY AREA
2.1 Location of the study area
A large part of the study area falls within the boundaries of Umziwabantu Local Municipality of the Ugu District Municipality. The Bongwana fault is located on the border between KwaZulu Natal and the Eastern Cape (Figure 2-1). The fault is approximately 80 km long and runs from west of Port Shepstone in KZN to the south of Bizana in the Eastern Cape. Three sites along the fault, where CO2 gas issues from the fault gouge, have been identified. These sites are known as Umzimkulwana River, Mbangweni River and Umtamvuna River.
Umzimkulwana River CO2 emission site: In this site, CO2 issues from an approximately 20 m wide fault gouge within the beds of Umzimkulwana River. CO2
samples obtained by Young (1923) and reported in Gevers (1941) indicates CO2
percentages of 98.3% and 97.6% for 2 samples.
Mbangweni River CO2 emission site: At this site, CO2 bubbles from the river beds at the position were the Bongwana fault cuts through the river, however, the bubbles are very small.
Umtamvuna River CO2 emission site: The CO2 springs near Umtamvuna River are associated with the deposition of travertine cones with groundwater at low temperatures between 20 and 22 degrees Celsius.
Another site where CO2 issues from a borehole drilled for water supply occurs parallel to the main fault. This site is believed to be associated with a minor faults parallel to the main fault. This site is referred to as “Farm Lot 4”.
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Figure 2-1: Locality map of the study area
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2.2 Climate and drainage
The climate of the study area is characterized by warm summers and cool winters.
Precipitation in the study area is seasonal, with most rainfall occurring as thunderstorms during the period between October and March. The peak rainfall months are December to January (Figure 2-1). The rate of rainfall increases with increasing altitude, generally from east to west and the mean annual precipitation ranges between 700-1350 mm.
Figure 2-2: Average monthly rainfall for the study area.
The drainage of the study area falls within the Mvoti to uMzimkhulu Water Management Area (tertiary catchments T52 and tertiary catchment T40) water resource zone. All the rivers within the study area drain to the south, into Umtamvuna River which forms the southern part of the study area, and ultimately flows into the Indian Ocean (Figure 2-3).
0 20 40 60 80 100 120 140 160
Precipitation (mm)
Time (months)
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Figure 2-3: Drainage map of the study area
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2.3 Geological setting
The regional geology of the Bongwana region is represented by lower Karoo Dwyka Group rocks made up of diamictites, tillites and minor shales, sandstones and conglomerates, which are typical fluvio-glacial environments belonging to the late Carboniferous to early Permian Dwyka Group. Figure 2-4 is a generalized geological map of the study area. The Dwyka Group in the south of the study area rest unconformably or paraconformably on the Cape Supergroup while on the east it unconformably overlies the Msikaba Formation and Natal Group (Johnson et al., 1997).
The deposition of these Dwyka sediments took place when Gondwana migrated over the South Pole during the Carboniferous age (Botha, 1998). The diamictites of the Dwyka Group are highly compacted and generally consists of angular to rounded clasts of the basement rocks embedded in a clay and silt matrix.The sandstones are generally very fine to medium-grain, massive to ripple-laminated, or medium to coarse grain, trough cross-bedded and immature (Von Brunn, 1994).
The rocks of the Msikaba Formation underlying the Dwyka Group are mainly quartz- rich sandstones with intercalated “grit” and conglomerate layers and lenses (Marshal, 1999). The Msikaba Formation was deposited in high-energy shallow-marine environment on a stable platform (Kingsley, 1975). The Msikaba Formation is mainly present in the Kwazulu-Natal province section of the study area. On the Eastern Cape Province side, the Dwyka Group overlies the Table Mountain sandstones (TMS).
The rocks of the Msikaba Formation and the TMS are underlain by basement rocks belonging to the Natal sub-province of the Namaqua-Natal Belt. The Natal Sub- province is divided into three Terranes, namely: Tugela, Mzumbe and Margate Terranes. The Tugela and Mzumbe Terranes are situated to the north of the study area, while the Margate Terrane occurs in the study area. The oldest rocks of the Margate Terrane comprises of the supra-crustal gneisses of the Mzimkulu Group, which the South African Committee on Stratigraphy (SACS, 1980) subdivided it into the Leisure Bay Formation, the Marble Delta Formation and the Mucklebraes Formation.
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The Leisure Bay Formation is dominated by meta-pelitic rocks with subordinate calc- silicate gneisses and is described in detail by Grantham (1984). The meta-pelitic gneisses comprise equigranular, finely banded, biotite–orthopyroxene–garnet gneiss and medium-grained melanocratic, biotite – orthopyroxene – garnet – cordierite gneiss (Mendonidis and Grantham, 2003). The cordierite-bearing gneiss occurs as 50 cm bands within the more common biotite – orthopyroxene – garnet gneiss. The calc- silicate rocks occur as thin, homogeneous layers of diopside, sphene and plagioclase intercalated with the meta-pelites (MacCourt et al., 2006).
The Marble Delta Formation is dominated by carbonate rocks and is described in detail by Otto (1977), who recognises an upper, calcitic ‘‘Oribi’’ member and a lower, dolomitic ‘‘Le Jonquet’’ member. The upper member consists mainly high grade calcite that is exploited for commercial purposes (MacCourt et al., 2006). Common accessory minerals include forsterite, graphite, phlogopite, scapolite and serpentine.
Subordinate amphibolite forms a relatively thin zone structurally overlying the calcite-rich upper member (MacCourt et al., 2006). Veins of porphyritic charnockite thought to be part of the Oribi Gorge Suite intrude the upper unit, and wollastonite is locally developed along the intrusion contacts. The lower unit is made of dolomite that is currently being commercially exploited for cement production. It contains diopside, altered to tremolite, and forsterite, altered to serpentine, as common accessory minerals (MacCourt et al., 2006).
The Mucklebraes Formation comprises mafic granulite and calc-silicate rocks (Thomas, 1988). The mafic gneisses are two-pyroxene granulites with calcic plagioclase, biotite and hornblende whereas the calc-silicate rocks carry diopside, garnet and idocrase (McCourt et al., 2006).
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Figure 2-4: Simplified Geology map showing the prevailing geological conditions within the study area (Council for Geosciences, 1988)
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2.4 Hydrogeological conditions
The hydrogeological conditions within the study area are controlled by the geology, structure and climate. The surface geology within the study area is dominated by rocks of the Dwyka Group, which is composed of mainly massive tillites, diamictites and minor sandstones and shales. The Dwyka Group rocks exhibit no primary porosity and generally display a very low hydraulic conductivity (WRC, 2002). However, these rocks may exhibit secondary porosity in areas where they have been intruded by Karoo dolerites or when affected by tectonic activity such as faulting.
Underlying the Dwyka Group rocks within the study area are the rocks of the Msikaba Formation. Msikaba Formation comprise the light coloured quartz arenite rocks at its upper part and pale-brownish conglomerate and sandstone at its lower part.
Cementation and compaction have decreased the primary porosity in the rocks.
Groundwater in the Msikaba Formation is mainly hosted by the secondary porosity.
2.4.1 Aquifer types
The aquifer within the study area is classified as an intergranular and fractured aquifer system with low primary porosity. Groundwater is hosted by secondary porosity created due to fracturing and faulting. According to the hydrogeological map of Durban (King, 1998), the study area is characterized by borehole yields ranging from 0.1 to 0.5 l/s.
Since the sandstones associated with the Dwyka Group were deposited mainly under marine conditions, water from these sandstones tends to be saline with EC values exceeding 1000 mS/m (King, 1998). Exploitable aquifers in the Dwyka Group exist in few places, where sand and gravel are dominant or where the Dwyka Group rocks are extensively intruded by dolerite dykes or fractured. Groundwater recharge in the Dwyka Group is very small ranging from 2 to 3 percent of the mean annual precipitation (MAP). Therefore, based on Parsons’ (Parsons, 1995) aquifer classification system, the rocks underlying the study area can be classified as minor aquifers.
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2.4.2 Groundwater levels and groundwater flow direction
Groundwater level data for the study area was obtained from the Department of Water and Sanitation (DWS) KZN_GRIP database. Groundwater levels range between 10 and 100 m bgl. Boreholes in the northern part of the study area, near Bongwana Station display shallow groundwater levels (10 to 50 m bgl) and boreholes to the southern part of the study area show deep groundwater levels (50 to 100 m bgl). The groundwater level data indicates a strong correlation between groundwater elevation and topographic elevation (Figure 2-5). It is therefore assumed that groundwater flow within the study area mimics the topography. Groundwater level data used to construct the groundwater level elevation is in Appendix A.
Figure 2-5: Groundwater level elevation vs altitude
2.4.3 General groundwater quality
The available groundwater quality data from the GRIP boreholes suggest that the highest EC and TDS values are 100 mS/cm and 700 mg/l, respectively. The pH values vary between 7 and 8, with most boreholes displaying neutral pH conditions. Trace element concentrations appear to be low throughout the study area. Sodium and calcium appear to be the dominant cations.
R² = 0.88
500 550 600 650 700 750 800 850
400.00 450.00 500.00 550.00 600.00 650.00 700.00 750.00 800.00 850.00
Altitude (mamsl)
WL Elevation (mamsl)
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3. CHAPTER 3: LITERATURE REVIEW
3.1 Carbon dioxide emissions and climate change
Carbon dioxide is one of the main Greenhouse gases and is the primary contributor in the recent global climate change (IPCC, US EPA). CO2 is absorbed and emitted naturally as part of the carbon cycle, through plant and animal respiration, volcanic eruptions, and ocean-atmosphere exchange. As part of the natural carbon cycle, people and animals breathe in oxygen from the air and breathe out CO2. On the other hand, green plants absorb CO2 for photosynthesis and emit oxygen back into the atmosphere.
As a greenhouse gas, its presence in the atmosphere traps heat from the sun. Normally, this keeps the climate warm enough for life to continue. Human activities, such as the burning of fossil fuels and changes in land use, release large amounts of CO2, which result in the increase in the levels of CO2 in the atmosphere. The increase in the CO2
levels in the atmosphere contributes to global climate change (IEA, 2008).
There are numerous techniques that are currently explored to determine ways of curbing the CO2 emissions into the atmosphere. One of the techniques that are currently perceived as feasible in curbing the CO2 emissions is the Carbon Capture and Storage (CCS) in deep geologic formation. This technique is widely studied throughout the world. South Africa is looking at employing this technique as an effort to curb the CO2
impacts in the atmosphere. Two sites in South Africa have been identified with the potential of storing CO2.
The primary site is the Zululand Group, in the northern KwaZulu Natal areas known as Kwamhlabuyalingana area. The second site is the Algoa Bay in Eastern Cape. The primary concern with the CCS technique in South Africa is lack of monitoring experience since the project is still in a testing phase, hence the Bongwana monitoring project was initiated to counteract this challenge. The following sections elaborate more on the sources of CO2, its impact on groundwater and CCS technique.
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3.1.1 Source of CO2
Carbon dioxide gas can be derived from both anthropogenic and natural processes by the burning or chemical treatment of organic matter, materials of organic derivation such as coal, oil and the hydrocarbon gases and rocks composed of carbonate minerals.
Anthropogenic sources
According to the Environmental Protection Agency (EPA) in the United States, The main anthropogenic sources of CO2 is the combustion of fossil fuels (coal, natural gas, and oil) for energy and transportation, although certain industrial processes and land- use changes also emit CO2. The combustion of fossil fuels to generate electricity is the largest single source of CO2 emission in the nation. The type of fossil fuel utilized to produce electricity will release different amounts of CO2. The burning of coal to generate electricity will produce more CO2 than oil and gas. The combustion of fossil fuels such as gasoline and diesel to transport people and goods also emit significant amounts of CO2 into the atmosphere. In addition to the energy and transportation sectors as sources of CO2, many industrial processes emit CO2 through fuel combustion, however, there are also other processes, apart from combustion, that produces CO2. These processes emit CO2 through chemical reactions that do not involve combustion, for example the production and consumption of mineral products such as cement, the production of metals such as iron and steel and production of chemicals (EPA, 2016).
Natural sources
The generation of CO2 by natural processes occur in a similar manner in which the generation of CO2 by anthropogenic sources occurs. The generation of CO2 by natural processes takes place when natural materials containing carbon are subjected to magmatic assimilation, heat generated by faulting, igneous intrusion and metamorphism, the action of acid groundwater on carbonate rocks and the kinds of decay and fermentation that occur during the transformation of buried organic matter into coal and hydrocarbons. Some of the most important natural processes responsible for the generation of CO2 in the earth’s crust are summarised below:
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Degassing of magma: most of the naturally occurring CO2 emitted from the earth crust originates from the degassing of magma. Magma degasses due to the pressure reduction that occurs when magma rises to the earth’s surface. This enables the dissolved CO2
and other gases such as water vapour to be released from the solution in the magma and accumulate as free gas. Most of the CO2 originating from the degassing of magma is released through volacanoes and associated fissures or hydrothermal sites such as one Yellowstone National Park in the USA (BGS, 2005).
Contact metamorphism of carbonate rocks: magma does not always daylight onto the earth surface. Sometimes it intrudes into the sedimentary rocks and crystallises at depth as plutons or dykes and sills. The heating and metamorphism of carbonate rocks by magmatic intrusions may result in carbonate rocks being metamorphosed into oxides and hydroxides and will give off CO2, this process is similar to the calcination of limestone in cement manufacture. The process of heating and metamorphism of the host rock by intrusions is known as contact metamorphism.
Thermal maturation of type III (coaly) kerogen and coals: Kerogens are organic chemical compounds generated from the plant and animal organic matter that becomes incorporated in sediments. Rocks with elevated quantities of kerogen are known as petroleum source rocks. Most source rocks are mudstones and shales that contain a few amounts of finely dispersed kerogen while some source rocks, known as oil shales may contain large amounts of kerogen. As kerogen rich rocks become buried deep in the earth crust in actively subsiding sedimentary basins, they are converted into other compounds by increasing heat and pressure during a process called maturation. During maturation process, kerogen-rich rocks give off volatiles such as water, methane, CO2
and the compounds that make up crude oil.
Biogenic Breakdown of oil and gas: Carbon dioxide may also be formed by the breaking down of oil and gas. The good example of this is the oil that is leaking from a salt diaper in the Central North Sea is biodegrading as it moves upwards and converted to carbon dioxide (Cody et al., 1999; Clayton et al., 1997). δ13 Cco2 values vary between -43.9 to -34‰ which is indicative of biodegradation process (BGS, 2005).
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Regional metamorphism of carbonate rocks: Regional metamorphism affects broad areas of the crust, such as places where plates once collided and mountain belts formed.
It results from the burial of rocks to sufficient depths within the earth for the temperature and pressure to change their mineral composition. It has been proposed that regional metamorphism of limestone could result in the generation of CO2 in a similar manner to contact metamorphism.
3.1.2 Migration of CO2 in the earth’s crust
Carbon dioxide formed at depth beneath the earth’s crust tend to migrate upwards towards the earth’s surface because it is lighter (BGS, 2005). Most of the CO2 generated in natural systems does not encounter suitable subsurface structures that could trap it and so it is able to migrate both laterally and vertically along permeable pathways.
These pathways could be layers of porous and permeable sedimentary rocks, such as sandstones, and/or fractures and fissures that cut through both permeable and otherwise less permeable rocks. The migration of CO2 in the rocks is in many ways similar to that of natural gas such as methane. When CO2 gas permeates through the rocks gas flow tend to be concentrated along any fault or fissures that may be present, or along the outcrop of porous and permeable sedimentary rocks. (BGS, 2005). Most rocks at shallow depths of a few tens of meters or less contain fractures such as joints and faults that are open and highly permeable.as a result, in many natural CO2 emission sites the CO2 tends to flow along these fractures. Although CO2 does not necessarily escape along the entire length of a fracture or fault, it will tend to emerge at one or more discrete points along the fault or fracture. The reason for this is that the permeability along the fault length varies and once the breakthrough occurs at one point, a channeling effect will occur.
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In an offshore environment, the movement of gases through the sea bed commonly produces pits called “pockmarks. In some instances, CO2 emerges at the sea bed dissolved in water or as a free gas. If in a free gas phase it may form a train of bubbles that will rise through the water column. In onshore sedimentary basins, migrating CO2
will pass through two hydrogeological zones on its way to the surface. Most of it passage will be through the lower saturated zone, below the water table, where pore spaces, and any fractures found within the rocks are fully saturated with water. The unsaturated zone above the water table is largely filled with soil gas. Once CO2 emerges through the saturated zone it will tend to disperse within the unsaturated zone. It may pool on top of the water table and disperse laterally before emerging at the ground surface.
The other way in which naturally occurring CO2 typically appears from the ground in onshore sedimentary basins is in carbonated springs. These occur when CO2 has dissolved in groundwater in the saturated zone. The example of such springs are found in France. In some countries, these have been used as sources of drinking water.
3.1.3 Impacts of dissolved CO2 on water chemistry
Studies of the impacts of CO2 from the CO2 sequestration sites reveals that the dissolution of CO2 in groundwater to form carbonic acid and its subsequent dissociation causes a decrease in pH. Acidic pH conditions in groundwater promotes the dissolution of carbonate rocks such as calcite according to the following mass balance reactions:
CO2(g) = CO2(aq) (1)
CO2 (aq) + H2O = HCO3- + H+ (2)
CaCO3 (calcite) + H+ = Ca2+ +HCO3- (3)
Changes in pH and the production of HCO3- will influence or control the dissolution of minerals and the subsequent release of chemical elements and contaminants into the aqueous phase, as well as precipitation reactions and formation of neophases. In addition, these changes may significantly affect the extent and rate of chemical, biological, and hydrological processes and reactions, which may control contaminant mobility in the subsurface.
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Experimental and modeling studies indicate that CO2 intrusion into the vadose zone or potable aquifers could induce a decrease in aqueous pH of 1 to 3 units (Little and Jackson, 2010; Lu et al., 2010; Wang and Jaffe, 2004; Wilkin and DiGiulio, 2010;
Zheng et al., 2009; Kharaka et al., 2010). Decreases in pH well-buffered systems where CO2-induced dissolution of reactive carbonates (Equation 4), feldspars (Equation 5), and/or the dissolution/precipitation of clays (Equation 3 and 4) provide enough buffering capacity (via HCO3- alkalinity) to resist drastic changes in pH.
CaCO3 + CO2 (g) + H2O → Ca2+ + 2HCO3- (4) 2NaAlSi3O8 + 11H2O + 2CO2 (g) → Al2Si2O5 (OH)4 + 2Na+ + 2HCO3- + 4H4SiO4 (5) Al2Si2O5 (OH)4 + 5H2O + 6CO2(g) → 2Al3+ + 6HCO3- + 2H4SiO4 (6) This suggest that rocks containing carbonate minerals can effectively buffer the acidity associated with the dissolution of CO2 in groundwater. The dissolution of calcite takes place as long as there is enough CO2 dissolving in groundwater. When there is no more dissolving CO2 in groundwater, groundwater eventually becomes saturated with respect to calcite and will no longer be able to dissolve the rock. Furthermore, when the CO2- rich groundwater gets into the surface, CO2 is released into the atmosphere and the dissolution of calcite drops. When the CO2 is lost, the solubility of calcite drops and calcite crystallizes from water. This is the mechanism by which specleothems (Stalactites etc) grow within caves, or travertine (calcite crusts) are formed at surface.
Poorly buffered systems (e.g., sandy soils in aquifers) are devoid of sufficient quantities of alkalinity producing minerals and therefore lack the ability to resist changes in pH.
In such systems, the decrease in pH due to the dissolution of CO2 into solution is generally more apparent and the risk for pH-induced perturbation to environmental quality is more significant and prolonged compared to well-buffered systems (McGrail et al., 2006; Wang and Jaffe, 2004; Wilkin and DiGiulio, 2010).
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3.1.4 Technique to curb CO2 release: Carbon Capture and Storage (CCS) CCS is defined as the process of capturing CO2 from large emission sources and injected, in a supercritical state, normally into deep geological formations that are capable of storing the CO2 almost permanently without release from the storage formation (Lemieux,2011) and has been deemed as a promising alternative for reduction of greenhouse gas emission. Successful geologic storage of CO2 requires storage capacity with high effective porosity in the geological formation and should be feasible of high injection rate of CO2. Concurrently, there should be a confining layer on top of the permeable geological formation to prevent CO2 from migrating outside of the rock formation. The existence of this confining layer on top of the permeable saline aquifer is primary requirement for the CO2 storage facility in deep geological formations. For successful confinement, the injected CO2 should be trapped physically, chemically, or mineralogically.
The fate of the injected CO2 and its effects on the chemical changes of groundwater quality have been extensively proposed and discussed through laboratory (Little and Jackson, 2010; Lu et al., 2010; Frye et al., 2012; Terzi et al., 2014) and field tests (Kharaka et al., 2010; Cahill et al., 2014). It has been understood that dissolved CO2 in groundwater has potential to degrade the shallow depth groundwater quality (Bachu, 2008; Newmark et al., 2010; Harvey et al., 2012; Siirila et al., 2012). Thus, although it is challenging, development of detection system in a shallow aquifer region is crucial to assure the long term safety of the injected CO2 in deep saline aquifers. The previous CCS cases show that continuous monitoring of pH, EC, TDS and major cations and anions concentration is the most basic and important factor for leakage detection. Trace elements and isotopes are also widely used to determine the CO2 leakage.
The integrated technological CCS process consists of the separation of CO2 from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere. There are three main approaches that are employed in CCS, namely: Pre-combustion, Post-combustion and Oxyfuel combustion (IPCC, 2005). Figure 3-1 below shows schematic representation of the processes involved in the CCS.
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1) Pre-combustion
Pre-combustion CCS takes place before the fuel (normally coal or natural gas) is placed in the furnace by first converting coal into a clean-burning gas and separating the CO2
released by the process. For coal, the pre-treatment involves a gasification process conducted in a gasifier under low oxygen level forming a syngas which consists mainly of CO and H2, and is mainly free from other pollutant gases.
The syngas will then undergo water gas shift reaction with steam forming more H2
while the CO gas will be converted to CO2. The following reaction equations illustrate the process of CO2 separation by pre-combustion method.
𝐶𝑜𝑎𝑙 𝐺𝑎𝑠𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛
⇒ 𝐶𝑂 + 𝐻2 (7)
𝐶𝑂 + 𝐻2𝑂 𝑊𝑎𝑡𝑒𝑟−𝐺𝑎𝑠 𝑆ℎ𝑖𝑓𝑡
⇒ 𝐻2+ 𝐶𝑂2 (8)
Natural gas, as it mainly contains CH4, can be reformed to syngas containing H2 and CO.
𝐶𝐻4 + 𝐻2𝑂 𝑅𝑒𝑓𝑜𝑟𝑚⇒ CO + 𝐻2 (9)
The content of H2 can be increased by the water gas shift reaction (Eq. (8)) and the rest of the process is similar to that described above for coal.
Figure 3-1: Schematic representation of the processes involved in CCS.
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2) Post-combustion
In the post-combustion method, CO2 is separated from the flue gas of the power station by bubbling the gas through an absorber column packed with liquid solvents (such as ammonia) that preferentially take out the CO2. In the most commonly-used techniques, once the chemicals in the absorber column become saturated, a stream of superheated steam at around 120°C is passed through it. This releases the trapped CO2, which can then be transported for storage elsewhere.
3) Oxyfuel combustion
Oxyfuel combustion systems use oxygen instead of air for combustion of the primary fuel to produce a flue gas that is mainly water vapour and CO2. This results in a flue gas with high CO2 concentrations (greater than 80% by volume). The water vapour is then removed by cooling and compressing the gas stream. Oxyfuel combustion requires the upstream separation of oxygen from air, with a purity of 95–99% oxygen assumed in most current designs. Further treatment of the flue gas may be needed to remove air pollutants and non-condensed gases (such as nitrogen) from the flue gas before the CO2
is sent to storage. As a method of CO2 capture in boilers, oxyfuel combustion systems are in the demonstration phase (see Table TS.1). Oxyfuel systems are also being studied in gas turbine systems, but conceptual designs for such applications are still in the research phase (IPCC, 2005).
4) Transportation
When the storage site is not located directly next the emission sources, as in the case of South Africa, the captured CO2 needs to be transported. Pipelines have been used for this purpose in the USA since the 1970s. CO2 could also be transported in liquid form in ships similar to those transporting liquefied petroleum gas (LPG). For both pipeline and marine transportation of CO2, costs depend on the distance and the quantity transported. For pipelines, costs are higher when crossing water bodies, heavily congested areas, or mountains.
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5) Storage
CO2 can be stored into geological formations such as deep saline aquifers which have no practical use, and or gas reservoirs. Geological storage is at present considered to be the most viable option for the large CO2 quantities needed to effectively reduce global warming and related climate change (Leung, 2014). Deep ocean storage is also a feasible option for CO2 storage although there are environmental concerns such as ocean acidification and eutrophication that are most likely to limit its application. The research shows that deep saline aquifers can stored large amounts of CO2 gas than depleted oil and gas fields.
Studies in the Zululand basin are currently underway to assess the feasibility of storing CO2 in deep saline aquifers of the Zululand Group. The Zululand basin forms a potential onshore target for CCS in South Africa, located on the east coast of South Africa in the northern KwaZulu-Natal Province.
The basin represents an onshore extension of the southern Mozambique Basin, with basin-fill sediments of late Barremian to late Maastrichtian age (Chabangu et al., 2014).
The Zululand Basin comprises the rocks belonging to the Zululand Group. Six sandstone packages with varying reservoir properties were identified in the Zululand Basin (Chabangu et al., 2014), however due to depth restrictions for CCS only two were investigated. The basal, Aptian-aged sandstone identified in the Makatini Formation represents the lower potential reservoir whilst a sandstone succession of upper Cenomanian- to Turonian-age represents the uppermost sandstone unit of the Makatini Formation and the lowermost sandstone unit of the St Lucia Formation. Both reservoir packages are well developed at the Kosi Trough.
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3.2 Previous studies on the Bongwana fault
Natural CO2 springs at the Bongwana area were first studied in the early 20th century by Young (1923) and Gevers (1941). Further work conducted by Gevers (1941) and Du Toit (1946) led to the identification and characterisation of the other CO2 springs associated with the deposition of the travertine cones near Umtamvuna River, south of the Bongwana area. These CO2 springs were believed to be associated with an 80km long N-S trending fault known as Bongwana fault (Harris, et al., 1997). Young (1923) stated that CO2 gas emanates from fault fissures in the Dwyka tillites, indicated in the surface by silicified wall rock. Analysis of the fresh and kaolinised Dwyka tillites showed 65.49 % and 75.21% silica, 14.82% and 16.63% Al2O3 respectively.
Appreciable amounts of calcite and magnesium were also observed as CaO (2.99%) and MgO (2.79%). CO2 samples obtained by Young (1923) (published in Gevers 1941) returned CO2 percentages of 98.3% and 97.6% for 2 samples, each with O2 content of 0.2%, and N2 (by difference) of 1.5% and 2.2% respectively. The analysis of the carbon and oxygen isotopes for Bongwana CO2 gas exhalations done by Harris et al., (1997) from seven locations in the Bongwana area ranged from -0.98 to 0.85‰ for δ13C and from 35.31 to 45.06‰ for δ18O. The analysis of the travertine rocks samples near Umtamvuna River published by Gevers (1941) indicated high calcite (41.85%) as CaO and iron (14.64%) as Fe2O3. Appreciable amounts of silica (1.43%) and magnesium (1.73%) as MgO were also reported. Two models were proposed for the CO2 generation along the Bongwana fault. The model proposed by Gevers (1941) suggested that CO2
is generated at depth by the reaction of groundwater and carbonate rocks. The second model proposed by Hartnady (1985) suggest a magmatic source of the CO2. The carbon and isotope data published by Harris et al., (1997) for CO2 gas from the Bongwana fault indicated carbonate source for CO2 generation in favour of the Gevers model.
According to Gevers (1941) groundwater from the travertine cone springs originate from the Dwyka Group (local recharge) and percolate to greater depth. However, no isotope data to support this, the assumption was based on the fact that the two groundwater sources exhibit the same temperatures.
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4. CHAPTER 4: RESEARCH METHODOLOGY
The information presented in this research was generated during the course of the research field observations and testing, and also collected from various sources on a desktop level study. The information gathered was then analysed using dedicated hydrogeological software packages and presented in the form of tables, graphs and diagrams. A hydrogeological conceptual model was then proposed based on the results of the analyses of all information.
4.1 Data collection
The data in this study was collected by undertaking a desktop survey as well as a literature review of all available information regarding the research topic. The field assessment was undertaken to map the study area, conduct a hydrocensus and collect water samples for hydrochemical and isotope analysis. The field water quality parameters were measured during the field assessment.
4.1.1 Desktop study
The desktop study was undertaken by assessing all the available information on the topography, rainfall, drainage, geology and hydrogeology of the study area. The following data sources were used:
1:250 000 Geological Map Series: 3030 Port Shepstone, prepared by Council for Geosciences;
1:250 000 Geological Map Series: 3028 Kokstad, prepared by Council for Geosciences;
1:250 000 Geological Map Series: 3128 Umtata, prepared by Council for Geosciences;
1:500 000 Hydrogeological Map Series of the Republic of South Africa, Durban area, prepared by King (1998);
Google Earth images;
Department of Water and Sanitation (DW&S) 2016 KZN GRIP data;
CHART database compiled by the Department of Water and Sanitation.
